The document EP-A2-1 164 555 discloses a communication unit including a control device, which comprises a receiver to receive an enabling signal and a controller to enable operation of the communication unit in dependence upon the enabling signal. An active badge transmits the enabling signal. If the communication unit and the badge are separated and the communication unit is no longer able to receive the enabling, then the controller disables the communication unit. It is to be noted that the date of publication for this document is 2001 Dec. 19.
The document U.S. Pat. No. 5,635,897 discloses a mobile phone alarm which comprises a phone device to be attached to a mobile phone which repeatedly transmit a verification signal (an electromagnetic signal) of a limited power to be receivable within a limited, predetermined range. A body device is clipped or otherwise secured to the phone user, similar to a beeper, to receive the verification signal from the phone device. The body device generates an alarm signal once the user is at a distance from the mobile phone greater than the transmission range of the verification signal to notify the user.
The document JP-A-08044974 discloses a radio type portable forgetting machine, which consists of a pair of a transmitter and a receiver, and the transmitter amplifies a high frequency generated by a high frequency generating circuit by an amplifier and sends it as a radio wave from an antenna to the receiver. When a device using the same frequency is nearby, the frequency can be varied with a switch to prevent cross interference. The receiver confirms whether or not there is the radio wave from the transmitter and when the radio wave is ceased, a buzzer sounds; and the sent radio wave is caught and detected by a reception pact circuit, and a reception storage circuit stores whether or not there is a detection signal. Namely, whether or not the radio wave from the transmitter arrives is stored. The stored signal is supplied to a buzzer generating circuit, which makes the buzzer sound by confirming that the detection signal does not arrives.
The document WO-00/19390 discloses an alarm system for alerting a golfer about a forgotten club. A divot tool incorporates a radio transmitter powered by a rechargeable power source for transmitting a coded transmission. A radio receiver tuned to receive the coded transmission includes logic circuitry that generates an alarm signal whenever power associated with the coded transmission drops below a threshold level. One or more alarm devices are coupled to the logic circuitry to generate alarm(s) when the alarm signal is supplied thereto. A power source is coupled to the radio receiver for supplying power thereto and for supplying a charging current for the rechargeable power source via an electrical coupling. When a golfer places a club or other accessory on the ground, the divot tool is placed near the club. As long as the golfer remains within a prescribed range, no alarm will be activated. However, if the golfer moves further from the divot tool than the prescribed range, an alarm is produced. Alerted to his forgetfulness before leaving the vicinity, the golfer then simply retrieves the club and the divot tool.
None of the above cited documents discloses a complete solution to the problem of preventing loss or theft of e.g. a portable device. Another drawback with all but the last solutions given above is that they require the use of a mobile telephone.